Microarray technology has been widely used for genomics and proteomics research as well as for drug screening. Currently, the spot size in most microarrays is larger than one micron. The use of nanometric biomolecular arrays, with smaller spot sizes, will enable high-throughput screening of biomolecules—eventually at the single molecule level. Also, nanometric arrays permitting precise control over the position and orientation of individual molecules will become a powerful tool for studying multi-valent and/or multi-component molecular interactions in biological systems. Toward these ends, protein arrays with feature sizes smaller than 100 nm have been fabricated, mostly using dip-pen nanolithography and nanografting. [See Lee et al., Science 2002, vol. 295, p. 1702; Wilson et al., Proc. Natl. Acad. Sci. USA 2001, vol. 98, p. 13660; Liu et al., Proc. Natl. Acad. Sci. USA 2002, vol. 99, p. 5165; Pavlovic et al., Nano Lett. 2003, vol. 3, p. 779; and Krämer et al., Chem. Rev. 2003, vol. 103, p. 4367.]
Modification of silicon surfaces with organic thin films to allow strong and highly specific interactions with targeted biological entities is of tremendous interest in the fields of biomicroelectrical mechanical systems (bioMEMS) that may integrate biosensing with controlled delivery of drugs. Target molecules interacting with the film surfaces can be detected optically, mechanically, magnetically, electronically or the combination. [Grayson, A. C. R.; Shawgo, R. S.; Johnson, A. M.; Flynn, N. T.; Li, Y. W.; Cima, M. J.; Langer, R. “A BioMEMS review: MEMS technology for physiologically integrated devices.” Proc. IEEE 2004, 92, 6-21.]
BioMEMS are of tremendous interest for their potential applications in microscale, high throughput biosensing and medical devices [Shawgo et al., J. Curr. Opin. Solid State Mater. Sci. 2002, v. 6, p. 329]. Using silicon as a substrate for the preparation of such devices is particularly attractive, since the extensive micro-fabrication techniques developed by the microelectronic industries can be used to fabricate and integrate various micro-components into the devices. For reducing biofouling, considerable research has been directed to the modification of substrate surfaces with stable and ultrathin films of poly(ethylene glycol) (PEG) or oligo(ethylene glycol) (OEG) [Prime et al., Science 1991, vol. 252, p. 1164]. Since many of the ultimate applications for bio-devices require moderate-term (e.g., a few hours to several days) exposure to biological media (e.g., buffer of pH 7.4 at 37° C.), stability of the bio-compatible coatings on the devices under these conditions is highly desirable. All of the OEG/PEG-terminated films on silicon substrates reported by others are bound onto the silicon surfaces via Si—O bonds that are prone to hydrolysis [Calistri-Yeh et al., Langmuir 1996, v. 12, p. 2747), thereby limiting their stability under physiological conditions (Sharma et al., Langmuir 2004, v. 20, p. 348].
For implantable bioMEMS, inflammatory responses often lead to device failure due to the formation of a thick layer of fibrous cells on the implant. [Wilson, G. S.; Gifford, R. “Biosensors for real-time in vivo measurements.” Biosens. Bioelectron. 2005, 20, 2388-2403.] The initial step of inflammatory response is the non-specific adsorption of proteins onto the substrates. This step alone may greatly lower the sensitivity and specificity of the implanted sensor. Therefore, ideal coating for silicon based biosensors should be 1) ultrathin for high sensitivity; 2) resistant to non-specific interactions with proteins and cells in body fluids and tissues; 3) strongly and specifically interacting with target molecules or cells; 4) stable over a period of time required by specific applications under in vivo conditions.
In addition to bioMEMS applications, ultraflat, stable and highly protein-resistant films on silicon surfaces represent ideal platforms for fabrication of single molecule arrays presenting signaling and adhesion molecules. These well-defined model systems allow for fundamental study of cell response to chemical signals at a single molecule level. A deeper understanding of how the nanoscale presentation of such molecules determine cellular functions, such as differentiation, proliferation and apoptosis, [Arnold, M.; Cavalcanti-Adam, E. A.; Glass, R.; Blummel, J.; Eck, W.; Kantlehner, M.; Kessler, H.; Spatz, J. P. “Activation of integrin function by nanopatterned adhesive interfaces.” Chem Phys Chem 2004, 5, 383-388; Maheshwari, G.; Brown, G.; Lauffenburger, D. A.; Wells, A.; Griffith, L. G. “Cell adhesion and motility depend on nanoscale RGD clustering.” J. Cell Sci. 2000, 113, 1677-1686.] is of tremendous importance for designing the next generation biomaterials, implantable bioMEMEs and pharmaceuticals.